Many circuits, networks, electrical devices and data handling systems are operated in configurations and environments where external factors can impair their performance, cause failure or even result in permanent damage. Among the most common of these factors are over-voltage and over-current. Protection against these factors is important and has been addressed in the prior art in various ways.
Fuses that employ thermal or magnetic elements are one common protection measure. In other cases, protection circuits are available. Some examples are described in U.S. Pat. Nos. 5,130,262; 5,625,519; 6,157,529; 6,828,842 and 6,898,060. Protection circuits are further specialized depending on conditions and application. For example, in the case of protecting batteries or rechargeable elements from overcharging and over-discharging one can refer to circuit solutions described in U.S. Pat. Nos. 5,789,900; 6,313,610; 6,331,763; 6,518,731; 6,914,416; 6,948,078; 6,958,591 and U.S. Published Application 2001/00210192. Still other protection circuits, e.g., ones associated with power converters for IC circuits and devices that need to control device parameters and electric parameters simultaneously also use these elements. Examples can be found in U.S. Pat. Nos. 5,929,665; 6,768,623; 6,855,988; 6,861,828.
When providing protection for very sensitive circuits, such as those encountered in telecommunications the performance parameters of the fuses and protection circuits are frequently insufficient. A prior art solution embodied by transient blocking units (TBUs) that satisfy a number of the constraints is considered in international publications PCT/AU94/00358; PCT/AU04/00117; PCT/AU03/00175; PCT/AU03/00848 as well as in U.S. Pat. Nos. 4,533,970; 5,742,463 and related literature cited in these references.
In a TBU, two or more transistors are arranged such that they normally provide a low series resistance. However, when an over-voltage or over-current transient is applied to the TBU, the transistors switch to a high impedance current blocking state, thereby protecting a load connected in series to the TBU. Variations and/or refinements of the basic TBU concept are considered in U.S. Pat. Nos. 3,916,220, 5,319,515, 5,625,519, 5,696,659, 5,729,418, 6,002,566, 6,118,641, 6,714,393, 6,865,063, and 6,970,337
In conventional TBU operation, the terminal voltages necessary to drive the TBU into its high impedance state are generated by the flow of current through the TBU transistors. Once these terminal voltages reach a predetermined threshold level (typically on the order of 1 V), the TBU switches to its current blocking state. The current required to bring the voltage to this level is the TBU trigger current. This feature of conventional TBU operation has undesirable consequences in certain cases. More specifically, an undesirably high TBU series resistance is required in cases where a low TBU trigger current is required. For example, a conventional TBU having a trigger current of 30 mA will need a series resistance of about 30Ω in order to switch a TBU having a voltage threshold of about 1 V. In normal operation, the 30Ω TBU series resistance undesirably increases power dissipation without providing any benefits. A conventional TBU having a 1Ω series resistance and a 30 mA trigger current would require a voltage threshold on the order of 30 mV, which is impractically low. It is apparent that the series resistance of a conventional TBU undesirably increases as the TBU trigger current decreases, since practical voltage thresholds cannot be substantially less than about 1 V.
Thus it would be an advance in the art to provide TBUs having reduced series resistance, especially in cases where the TBU trigger current is low.